Oxide dispersion strengthened (ODS) alloys are recognized to possess superior properties for high temperature applications; especially ODS alloys formed of superalloy materials. ODS alloys are distinguished from conventional alloys by the presence of dispersoids of fine particles and by an elongated grain shape which generally develops during a recrystallization heat treatment and/or hot and cold working. This particular grain structure enhances the high temperature deformation characteristics of ODS alloys by inhibiting the accumulation of inter-granular damage. As result of this and other properties, components fabricated from ODS alloys exhibit improved high-temperature creep strength and improved oxidation resistance as compared to conventional alloys.
However, ODS alloys are very difficult to weld and repair by conventional techniques (e.g., gas tungsten arc welding, laser welding, electron beam welding, etc.). Such fusion welding causes significant loss of strength. The alloys are furthermore difficult and uneconomical to process by less traditional processes such as friction welding.
ODS alloys are typically produced by mechanical alloying (MA) processes in which various metals, alloys and oxides are blended and ball mill processed to impact, smear and shear the powders to create particles consisting of all constituents. The particles are then packaged and extruded, or hot isostatically pressed (HIP) to achieve a desired shape. The very fine-grained resulting product with small oxide particles dispersed as dispersoids and with directional stresses is then heat treated to recrystallize and grow large directional grains and/or is processed by additional hot and cold working. Further improved strength properties can then be obtained by performing a secondary recrystallization heat treatment. This heat treatment consists of annealing at a defined temperature, which depends on the composition of the alloy, to increase the size of grains in the alloy. A successful secondary recrystallization heat treatment causes abnormal grain growth to produce course, anisotropic grains having elongated shapes that resist slip at the grain boundaries.
Mechanical alloying suffers from a number of problems. First, the success of the heat treatment (secondary recrystallization) for one alloy specimen often does not guarantee success for another specimen—even for alloys having identical compositions. So, yield and predictability is poor. Another problem relates to the difficulty of dispersing certain oxides (such as yttria) which can lead to excessively long milling times or inhomogeneous microstructures. Furthermore, mechanical alloying is often not suitable for mass-produced and/or large-size products containing ODS alloys because of complicated manufacture procedures as well as prohibitively high costs. Nickel based ODS alloys are especially difficult to cold work and to successfully recrystallize.
Additional challenges associated with ODS alloys involve general shaping and joining of these materials. Shaping and joining techniques which preserve the microstructure and intrinsic strength of ODS alloys are severely limited, which often curtails their ability to be incorporated into high-temperature, load-bearing structures. For example, excessive heating of ODS alloys can cause the oxide dispersoids to coalescence leading to severe agglomeration such that the dispersoids may no longer be effective in resisting slip at the grain boundaries. Melting of ODS alloys also results in “slagging off” of the oxide dispersoids reducing their strengthening ability. Since most ODS alloys derive their strength from an elongated grain structure, such disruption of the grain structure ultimately reduces creep strength.
Various attempts have been made to discover alternative techniques for producing ODS alloys which avoid the disadvantages described above. Park et al. (US 2013/0299470), for example, describe a process illustrated in FIG. 1 in which a laser beam 6 is employed to heat the surface 4 of a metal sheet or tube 2 to form a metallic matrix melt 8. In this process a nozzle 10 is used to propel a jet of oxide particles 12 contained within an inert carrier gas 14 into the matrix melt 8 to form, upon cooling with a lubricant or coolant, an ODS alloy layer 16 containing oxide dispersoids 18 which is bonded to the metal sheet or tube 2.
Although the process of Park et al. avoids some of the problems associated with mechanical alloying, it is limited to the formation of ODS alloy coatings upon metallic objects (i.e., sheets or tubes) whose surfaces can be readily melted using a laser beam. This process also requires the use of a lubricant or coolant to cool the molten ODS alloy matrix, and requires the use of an inert gas (i.e., Ar or He) to inhibit oxidation of the ODS alloy during the cooling process.
Funkhouser et al. (U.S. Pat. No. 5,449,536) teach development of an ODS coating on a substrate by spraying the ODS powder into a “hot zone” created by a laser above the substrate. The substrate is not melted but the partially melted or plasticized powder impinges on and adheres to the substrate. Again, the process is limited to coatings on metal objects and an inert process environment would likely be required.